1. Field of the Invention
This invention relates to a process for the production of polyethylene materials for fibers, films, sheets and the like of high mechanical strength and high elastic modulus.
2. Prior Art
Ethylene polymers of extremely high molecular weight of say one million or greater are generally known as ultrahigh molecular weight polyethylene hereinafter referred to as "UHMW polyethylene". Polyethylene of this type is in common use as an engineering plastic material characterized by high resistance to impact and to wear and also by peculiar self-lubrication. The polymer has been extensively applied to hoppers, silos, gears, linings and the like for use in various industrial sectors ranging from food processing, civil engineering, chemistry, agriculture and mining to backings for skiing plates and the like for use in sports and leisure supplies.
UHMW polyethylene if possibly highly oriented will provide stretched products that are superior in mechanical strength and elastic modulus. Such polymer, because of its high molecular weight, is literally too viscous for extrusion and orientation under usual molding conditions.
Japanese Patent Laid-Open Publication No. 56-15408 discloses that a gel resulting from a decalin dope of UHMW polyethylene is allowed to stretch-mold to give fibers of great strength and high elasticity. This dope however is rather low in polymer concentration, say 3 weight percent with a polymer of 1.5.times.10.sup.6 in weight-average molecular weight and one weight percent with a polymer of 4.times.10.sup.6. From the commercial point of view, such prior art method has much to be desired in that dope formation requires large amounts of solvents and meticulous attention in preparation and handling of highly viscous solutions.
To overcome or alleviate this problem, there have been proposed certain improved modes of molding as disclosed for instance in Japanese Patent Laid-Open Publication Nos. 59-187614, 60-15120 and 60-97836 and Preprints of the Society of High Polymers, Japan, vol. 34, p. 873 (1985), whereby UHMW polyethylene can be oriented at temperatures below its melting point as by extrusion, stretching or rolling. These methods involve diluting the polymer in xylene, decalin, kerosine or the like, followed by cooling or isothermal crystallization to form a single crystal mat which is then extruded and stretched in a solid phase. Such methods still leave the problem of exorbitant solvent consumption unsolved.
The present inventors, in an effort to overcome the foregoing difficulties of the prior art, have previously proposed, as disclosed in Japanese Patent Laid-Open Publication No. 63-66207, a process for producing a polyethylene material of great mechanical strength and high elastic modulus, which comprises drawing particulate polyethylene at temperatures below its melting point, the particulate polyethylene having an intrinsic viscosity in the range of 5-50 dl/g in decalin at 135.degree. C. and derived by poymerizing ethylene at a temperature below such melting point and in the presence of a catalyst comprising a solid component containing at least one of titanium and vanadium compounds and an organometallic compound.